Robust foreign objects detection

ABSTRACT

An apparatus and method for performing foreign object detection for a wireless power transmitter. A matching network and transmit coil are energized, and a resonance is excited. The resonance is allowed to decay. A temporal characteristic of the decay is measured. The temporal characteristic is analyzed to determine whether a foreign object is coupled to an electromagnetic field generated by the transmit coil.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/244,107, filed Aug. 23, 2016, entitled “ROBUST FOREIGN OBJECTSDETECTION,” which claims priority to U.S. provisional application Ser.No. 62/245,378, filed Oct. 23, 2015, titled “FOREIGN OBJECTS DETECTION,”and U.S. provisional application Ser. No. 62/245,381, filed Oct. 23,2015, titled “ROBUST FOREIGN OBJECT DETECTION,” each of which is herebyincorporated by reference in its entirety.

BACKGROUND 1. Technical Field

The techniques described herein relate generally to wireless powerdelivery, and particularly to detection of foreign objects in the fieldproduced by a wireless power transmitter.

2. Discussion of the Related Art

Wireless Power Transfer Systems (WPTS) are gaining increasing popularityas convenient way to deliver power without wires or connectors. WPTScurrently under development in the industry can be separated in twomajor classes: magnetic induction

(MI) systems and magnetic resonance (MR) systems. Both types of systemsinclude a wireless power transmitter and a wireless power receiver. Suchsystems can be used to power or charge mobile devices such assmartphones or tablet computers, among other applications.

Inductive WPTS typically operate in an allocated frequency range ofseveral hundred kilohertz using frequency variation as a power flowcontrol mechanism. MR WPTS typically operate on a single resonantfrequency using input voltage regulation to regulate output power. Intypical applications, MR WPTS operate at a frequency of 6.78 MHz.

Several industry committees have been working on developinginternational standards for consumer products based on wireless powertransfer.

SUMMARY

Some embodiments relate to a foreign object detection method for awireless power transmitter having a matching network and transmit coil.The method includes (A) energizing the matching network and transmitcoil and exciting resonance between the matching network and transmitcoil; (B) allowing the resonance to decay; (C) measuring a temporalcharacteristic of the decay; and (D) analyzing the temporalcharacteristic to determine whether a foreign object is coupled to anelectromagnetic field generated by the transmit coil.

Some embodiments relate to at least one non-transitory computer readablestorage medium having stored thereon instructions, which, when executedby a processor, perform the foreign object detection method.

Some embodiments relate to an apparatus for performing foreign objectdetection for a wireless power transmitter having a matching network andtransmit coil. The apparatus includes circuitry configured to: (A)energize the matching network and transmit coil and excite resonancebetween the matching network and transmit coil; (B) allow the resonanceto decay; (C) measure a temporal characteristic of the decay; and (D)analyze the temporal characteristic to determine whether a foreignobject is coupled to an electromagnetic field generated by the transmitcoil.

Some embodiments relate to an apparatus for driving a wireless powertransmitter and performing foreign object detection. The apparatusincludes a drive circuit configured to energize a matching network andtransmit coil of the wireless power transmitter, excite resonancebetween the matching network and transmit coil, and allow the resonanceto decay. The apparatus also includes a controller configured to measurea temporal characteristic of the decay and analyze the temporalcharacteristic to determine whether a foreign object is coupled to anelectromagnetic field generated by the transmit coil.

The foregoing summary is provided by way of illustration and is notintended to be limiting.

BRIEF DESCRIPTION OF DRAWINGS

In the drawings, each identical or nearly identical component that isillustrated in various figures is represented by a like referencecharacter. For purposes of clarity, not every component may be labeledin every drawing. The drawings are not necessarily drawn to scale, withemphasis instead being placed on illustrating various aspects of thetechniques and devices described herein.

FIG. 1 shows a block diagram of a wireless power system including awireless power transmitter and a wireless power receiver.

FIG. 2 shows a flowchart of a method of performing foreign objectdetection.

FIGS. 3A-3C show examples of a drive circuit implemented as class Damplifiers.

FIGS. 4A-4C show examples of a drive circuit implemented as class Eamplifiers.

FIG. 5 shows an example of wireless power reception circuitry for awireless power receiver .

FIG. 6 shows waveforms for an example in which stimulus is performed byswitching the inverter of FIG. 3C at a single switching frequency andsupply voltage VDC, with no wireless power receiver present.

FIG. 7 shows waveforms for an example similar to FIG. 6 in which awireless power recover is present in the field produced by the wirelesspower transmitter.

FIG. 8 shows an example of a stimulus that can fully charge therectifier filter capacitor Crec.

FIG. 9 shows an example of a double stimulus in which the switchingfrequency is changed.

FIG. 10 shows an example of a double stimulus in which the supplyvoltage is changed.

FIG. 11 shows an example of a double stimulus in which both theswitching frequency and supply voltage are changed.

FIG. 12 shows an example of continuous time measurements.

FIG. 13 shows an example of discrete time measurements.

DETAILED DESCRIPTION

Wireless power transfer can be degraded due to the presence of a foreignobject in the field produced by the wireless power transmitter.Conductive objects such as metallic objects may absorb power due to theinducement of eddy currents in the conductive object. The presence ofsuch an object can significantly degrade the efficiency of the wirelesspower transmission. If a metal object is present, efficiency may bereduced substantially (e.g., from 90% to 40%). Further, due to the powerabsorbed, the temperature of the object may increase significantly,which may be undesirable. Techniques have been developed for sensing thepresence of a foreign object. However, prior techniques for sensing thepresence of foreign objects have various disadvantages, includinglimited detection capability at low power levels, a lengthy detectionprocess which wastes power, and/or the need to add additional circuitryor coils which lead to additional complexity and expense.

The techniques and devices described herein enable the detection of aforeign object using relatively low power levels. In some embodiments,detection may be performed by energizing and controlling the drivecircuit of a wireless power transmitter, and measuring a characteristicof a transient in the wireless power transmitter. Based on the transientcharacteristic the wireless power transmitter can determine whether aforeign object is present in the field produced by the wireless powertransmitter. Advantageously, in some embodiments detection of a foreignobject may be performed without the need to add additional hardware.

FIG. 1 shows a block diagram of a wireless power system 100 including awireless power transmitter 1 and a wireless power receiver 11. Thewireless power transmitter 1 has a drive circuit 7 including an inverter3 that drives a transmit coil 10 through a matching network 6. Thewireless power transmitter 1 may include a regulated voltage source 2(e.g., a voltage regulator) that provides a regulated DC voltage to theinverter 3. The regulated voltage source 2 produces a regulated DCoutput voltage in response to control stimulus from the controller 5. Insome embodiments, the drive circuit 7 may be a class D or E amplifierthat converts the DC voltage at the input of inverter 3 into an ACoutput voltage to drive the transmit coil 10. Producing an AC outputvoltage enables wireless power transmission through electromagneticinduction. The controller 5 may control a signal generator 9 to drivethe inverter 3 with signals of a selected wireless power transmissionfrequency. As an example, the inverter 3 may be switched at a frequencybetween 100 and 205 kHz to transmit power to a wireless power receiverdesigned to receive wireless power according to the Qi specification forlow power Qi receivers and 80-300 kHz for medium power Qi receivers. Theinverter 3 may be switched at a higher frequency, such as a frequency ofgreater than 1 MHz, within an ISM band, e.g., 6.765 MHz to 6.795 MHz, totransmit power to a receiver designed to receive wireless power using MRtechnology. However, these frequencies are described merely by way ofexample, as wireless power may be transmitted at a variety of suitablefrequencies, in accordance with any suitable specification. Controller 5may be an analog circuit or a digital circuit. Controller 5 may beprogrammable, and may command signal generator 9 to produce signals at adesired transmission frequency based on stored program instructions, sothat inverter 3 switches at the desired transmission frequency. Matchingnetwork 6 may facilitate wireless power delivery by presenting asuitable impedance to the inverter 3. The matching network(s) may haveone or more capacitive or inductive elements or any suitable combinationof capacitive and inductive elements. Since the transmit coil 10 mayhave an inductive impedance, in some embodiments the matching network 6may include one or more capacitive elements, which, when combined withthe impedance(s) of the transmit coil 10, presents an impedance to theoutput of inverter 3 suitable for driving the transmit coil 10. In someembodiments, during wireless power transfer the resonant frequency ofthe matching network 6 may be set equal to or approximately equal to theswitching frequency of the inverter 3. The transmit coil 10 may berealized by any suitable type of conductors. The conductors may bewires, including solid wire or Litz wire, or patterned conductors, suchas patterned conductors of a PC board or an integrated circuit.

The AC current in the transmit coil 10 generates an oscillating magneticfield in accordance with Ampere's law. The oscillating magnetic fieldinduces an AC voltage into a receiver coil 12 of the wireless powerreceiver 11 in accordance with Faraday's law. The AC voltage induced inthe receiver coil 12 is provided through a matching network 13 to arectifier 14 that generates an unregulated DC voltage. Rectifier 14 maybe a synchronous rectifier or may be implemented using diodes. Theunregulated DC voltage is regulated using a DC/DC converter 15, theoutput of which may be filtered and provided to a load as output voltageVout. In some alternate embodiments the DC/DC converter 15 can bereplaced by a linear regulator or battery charger, or eliminatedaltogether.

As shown in FIG. 1, if a conductive foreign object 20 enters the fieldproduced by the transmit coil 10 of the wireless power transmitter 1,the wireless power transmission efficiency may be degraded and/or theconductive foreign object 20 may undergo significant heating. Examplesof conductive foreign objects 20 include coins, paperclips, keys, by wayof illustration.

According to the techniques described herein, the wireless powertransmitter 1 may be controlled to perform foreign object detectionprior to wireless power transmission. Performing foreign objectdetection allows the wireless power transmitter to determine whether ornot to perform wireless power transmission.

Foreign object detection may be performed as follows. When the wirelesspower transmitter 1 performs foreign object detection it may increasethe energy stored in one or more components of the matching network 6and/or transmit coil 10. A resonance in matching network 6 and/ortransmit coil 10 is excited and allowed to decay. A temporalcharacteristic of the decay of the resonance is measured. Since the rateof decay of the resonance is different depending on whether or not aforeign object 20 is present, the temporal characteristic of theresonance decay can be analyzed to determine whether or not a foreignobject 20 is present. Wireless power transmission can be enabled orinhibited based on this analysis. If a foreign object is determined tobe present, wireless power transmission can be disabled. If a foreignobject is determined not to be present, wireless power transmission maybe enabled.

FIG. 2 shows a flowchart of a method of performing foreign objectdetection, according to some embodiments. Such a method may be performedby the wireless power transmitter 1. Specifically, controller 5 may beconfigured to control performing the method. In step S1, the matchingnetwork 6 and/or the transmit coil 10 is energized. Step S1 may beperformed by increasing the energy stored in one or more passivecomponents in the matching network 6 and/or transmit coil 10. Matchingnetwork 6 and/or the transmit coil 10 may be energized by switchinginverter 3 while inverter 3 is powered by a suitable supply voltage.Examples of suitable switching frequencies and supply voltages arediscussed below. However, the network 6 and/or the transmit coil 10 neednot be energized by switching the inverter at a switching frequency. Toincrease the energy stored, a voltage may be applied across a capacitorof the matching network 6 to increase the energy stored in thecapacitor, a current may be applied to the transmit coil 10 which mayincrease the energy stored in its inductance, or the energy stored inboth may be increased. In some embodiments, when the wireless powertransmitter is energized in the foreign object detection mode it isenergized at a lower level than when in the wireless power transmitteris in the power transmission mode. A lower voltage and/or current may beapplied to the matching network 6 and/or the transmit coil 10 ascompared to the voltage and/or current applied in the power transmissionmode, which can limit the power consumed for foreign object detection.

The resonance may be excited by switching one or more switches of theinverter 3 into a state that causes a capacitor of the matching network6 to resonate with the inductance of the transmit coil 10. For example,the inverter may be switched at a suitable switching frequency. When theresonance is excited the capacitor of the matching network 6 exchangesenergy with the inductance of the transmit coil 10 at the resonantfrequency.

In step S2, the resonance between the matching network and transmit coilis allowed to decay. Energy transfer into matching network and transmitcoil may be inhibited in step S2, so that the matching network andtransmit coil can resonate freely without the addition of energy. As anexample, if step S1 includes switching the inverter 3 at a switchingfrequency, the switching may be stopped in step S2, and the inverterkept in a state that does not allow energy to flow into the matchingnetwork or transmit coil. For example, the output of the inverter may beheld in a low impedance state. The output voltage may be held constantat a fixed voltage (e.g., a common mode voltage such as ground, or VDC)by turning on the appropriate transistor(s) of the inverter. Theresonance is allowed to decay freely. If a foreign conductive object 20is present in the field produced by transmit coil 10, eddy currents areinduced in the object 20 which loads the resonant network formed by thematching network 6 and transmit coil 10, causing the resonance to decaymore rapidly than if no foreign object is present. Accordingly, thespeed of decay of the resonance is indicative of whether a foreignconductive object 20 is present.

In step S3, a temporal characteristic of the resonance decay may bemeasured. As should be appreciated, step S3 may be performed at leastpartially at the same time as step S2. To measure a temporalcharacteristic of the resonance decay, one or more measurements of thematching network 6 and/or transmit coil 10 may be made to detect howquickly the resonance changes. The measurement(s) may be made bycontroller 5, which may include suitable measurement circuitry, or aseparate measurement circuit. Any suitable parameters may be measured,such as the current or voltage, for example. As shown by the dashedlines in FIG. 1, the measurement(s) may be made at the matching network6 and/or the transmit coil 10.

In some embodiments, the decay may be exponential, and the speed of thedecay may be represented by a time constant. Determining the temporalcharacteristic may include measuring a time constant or a valueindicative thereof. In some embodiments, the temporal characteristic maybe determined by calculating a ratio of the currents or voltages as theydecay over time.

In step S4, the temporal characteristic may be analyzed to determinewhether it is indicative of the presence of a foreign object. In someembodiments, the quality factor Q of the wireless power transmitter 1may be determined based on the temporal characteristic and/or themeasurements themselves. As an example of the analysis that may beperformed in step S4, the temporal characteristic or quality factor Qmay be compared to data indicating expected temporal factors or qualityfactors Q. For example the wireless power transmitter 1 may store data(e.g., in non-volatile memory) representing quality factors Q of knownwireless power receivers. The quality factor Q determined from themeasured temporal characteristic may be compared with the stored data,and if it differs from the expected value(s) by more than a thresholdamount the measured quality factor may be indicative of the presence ofa foreign conductive object 20. As another example, the wireless powertransmitter 1 may receive data from the wireless power receiver 11indicating the quality factor Q of the wireless power receiver 11. Thequality factor Q determined from the measured temporal characteristicmay be compared with the received quality factor Q of the receiver, andif it differs from that of the receiver by more than a threshold amountthe measured quality factor may be indicative of the presence of aforeign conductive object 20.

In step S5, wireless power transmission by the wireless powertransmitter 1 may be enabled or inhibited based on the result of theanalysis. If the measured temporal parameter or quality factor Q isoutside of an acceptable range, wireless power transmission may beinhibited. If the measured decay is within an acceptable range, powertransmission may be enabled, and the wireless power transmitter 1 may beallowed to enter the power transmission mode. The quality factor Qconsidered acceptable may be based on quality factor provided by awireless power receiver to the wireless power transmitter via in-band orout-of-band communication

FIGS. 3A-3C show examples of drive circuit 7 implemented as class Damplifiers. FIGS. 3A and 3B show a single ended (half-bridge)configuration in which inverter 3 is implemented by transistors Q1 andQ2, matching network 6 is implemented by capacitor C_(RES). Transmitcoil 10 is represented by inductor L_(RES) and an equivalent seriesresistance (ESR). FIG. 3C shows a differential (full-bridge)configuration in which inverter 3 is implemented by transistors Q1-Q4,matching network 6 is implemented by capacitors C_(RES1), C_(RES2) andC_(RES3). The drive circuit 7 is powered by a DC supply voltage VDC.FIGS. 4A-4C show examples of drive circuit 7 implemented as class Eamplifiers.

FIG. 5 shows an example of wireless power reception circuitry for awireless power receiver 11. Matching network 13 is implemented by acapacitor C_(RES). Rectifier 14 is implemented by a full-bridge dioderectifier with an output filter capacitor Crec having a voltage Vrecacross it. DC/DC converter 15 is implemented by a post regulator/loadswitch that produces V_(out).

Having shown examples of drive circuit 7 and an example of wirelesspower reception circuitry for a wireless power receiver 11, examples ofways in which the method of FIG. 2 may be applied thereto will bedescribed.

Referring again to FIG. 2, and as discussed above, step S1 involvesincreasing the energy stored in matching network 6 and/or transmit coil10 and exciting their resonance. In the context of the drive circuits ofFIGS. 3 and 4, step S1 may include increasing the energy stored in anyone or more of the capacitive or inductive elements of the drive circuit7. Initially, the energy stored in drive circuit 7 may be zero. However,the techniques described herein are not limited to starting with zeroenergy stored in the drive circuit 7. In some embodiments, energy may betransferred to the drive circuit 7 by switching one or more transistorsof the inverter 3 to provide energy to the capacitor(s) and/orinductor(s) of the drive circuit 7 from the supply voltage VDC.

As an example, the switches of the inverter 3 may be switched at aselected switching frequency to transfer energy into the drive circuit7. The amount of energy transferred to the drive circuit 7 by switchingthe inverter 3 depends upon the magnitude of the supply voltage VDC, theswitching frequency, and the amount of time for which the switchingoccurs. In some embodiments, it is desirable to limit the amount ofenergy transferred to the drive circuit to limit power dissipation whenperforming foreign object detection. The amount of energy transferredmay be limited by setting VDC at a lower voltage during foreign objectdetection as compared to its value during power transmission.Alternatively or additionally, the switching frequency may be selectedto control the amount of energy transferred. The farther away theswitching frequency of the inverter 3 is from the resonant frequency ofthe drive circuit 7, the less energy will be transferred into the drivecircuit 7 per unit time. The amount of time for which inverter 3 isswitched also affects the amount of energy transferred. Reducing theamount of time for which inverter 3 is switched can reduce the amount ofenergy transferred to drive circuit 7. However, the techniques describedherein are not limited to transferring energy into the drive circuit 7by switching the inverter 3, as in some embodiments energy transfer tothe drive circuit 7 may be performed by connecting the passivecomponent(s) of drive circuit 7 to VDC (e.g., through inverter 3), or aseparate circuit may be used to provide energy to the passivecomponent(s).

FIG. 6 shows waveforms for an example in which step S1 is performed byswitching inverter 3 of FIG. 3C at a single switching frequency andsupply voltage VDC, with no wireless power receiver 11 present. In thisexample, VDC is 8V, which causes inverter 3 to produce a square wave of8 Vpp, as shown by waveform 61. In this example, the switching frequencyof the inverter 3 is 175 kHz. The switching of inverter 3 in step S1 isperformed for 206 microseconds. Then, S1 ends by stopping the switchingof inverter 3, and the resonance is allowed to freely decay in step S2.The current through inductor L_(RES) is shown as waveform 62. Thevoltage of node V_(res1) is shown as waveform 63. As can be seen fromwaveforms 62 and 63, the resonance decays freely in step S2 once thestimulus in step S1 is stopped.

FIG. 7 shows waveforms for an example similar to FIG. 6 in which awireless power receiver 11 is present in the field produced by thewireless power transmitter 1. The present inventors have recognized andappreciated that when a wireless power receiver 11 is present the decayof the resonance can vary depending on the state of charge of the filtercapacitor of the rectifier filter capacitor Crec (FIG. 5). If Crec isnot charged to a point where the diodes of the rectifier 14 arereverse-biased, the resonance at the wireless power transmitter 1 may beloaded by the wireless power receiver to charge Crec. This can affectthe rate at which the resonance of the transmitter decays, which mayaffect the measurement of the decay, and thus impact the accuracy offoreign object detection.

FIG. 7 illustrates this problem. FIG. 7 shows the stimulus waveform 71produced by inverter 3, waveform 72 showing the current through inductorL_(RES), waveform 73 showing the voltage of node V_(res1), waveform 74showing the current through rectifier filter capacitor Crec, waveform 75showing the voltage at the input of the rectifier 14, and waveform 76showing the voltage across the rectifier filter capacitor Crec. In thisexample, the rectifier filter capacitor Crec has a capacitor of 40 μF,by way of illustration. The stimulus waveform 71 frequency, voltage andduration are the same as that discussed above with respect to FIG. 6. Inthe example of FIG. 7, since the wireless power receiver is present therectifier filter capacitor Crec charges up during the period in whichthe stimulus waveform 71 is applied in step S1. The inventors haverecognized and appreciated that if capacitor Crec is not fully chargedby the end of step S1 it may continue to charge during step S2, whichmay load the decaying resonance at the transmitter and skewing themeasurement of the resonance decay. FIG. 7 illustrates in waveforms 76and 74 that the rectifier filter capacitor Crec is not fully charged bythe end of step S1, such that current continues to flow into therectifier filter capacitor Crec during S2, which may adversely affectthe measurement of the resonance decay.

FIG. 8 shows an example of a stimulus that can fully charge therectifier filter capacitor Crec prior to step S2. In this example, VDCis 8V, the switching frequency of the inverter 3 is 200 kHz, and step S1lasts 600 microseconds. FIG. 8 shows the stimulus waveform 81 producedby inverter 3, waveform 82 showing the current through inductor L_(RES),waveform 83 showing the voltage of node Vres1, waveform 84 showing thecurrent through rectifier filter capacitor Crec, waveform 85 showing thevoltage at the input of the rectifier 14, and waveform 86 showing thevoltage across the rectifier filter capacitor Crec. As shown, therectifier filter capacitor Crec can be fully charged before the start ofstep S2 by applying the stimulus for a sufficient duration. However, onedisadvantage of this approach is that it involves increasing the lengthof step S1, which may be inefficient, as power may be dissipated duringstep S1.

In some embodiments, the duration of step S1 can be reduced by applyinga sequence of inverter stimulus waveforms at different energy levels.The inverter stimulus waveform may have a period of time in whichrelatively high energy level is applied, followed by a period of timewith a lower energy level applied. Using a relatively high energy levelinitially allows charging the rectifier filter capacitor Crec quickly.Then, the energy level can be reduced to allow improved efficiency.

Applying a sequence of inverter stimulus waveforms can include applyinga “double stimulus” in which a first stimulus is applied in step S1 aand a second stimulus is applied in step S1 b, which may be at a lowerpower level than in step S1 a. However, the techniques described hereinare not limited to applying two different stimulus levels, as any numberof different stimulus levels may be applied.

As mentioned above, the stimulus applied step S1 a may be of a higherenergy level than the stimulus applied in step S1 b. The energy level isaffected by the voltage level VDC used to power the inverter 3, theswitching frequency, and the amount of time for which a stimulus isapplied. Increasing VDC or the amount of time for which the stimulus isapplied increases the amount of energy provided. A switching frequencyclose to the resonant frequency of the transmitter provides a higherenergy level than a switching frequency farther away from the resonantfrequency. Any combination of these parameters may be varied to adjustthe energy level applied in subsequent stimulus steps S1 a, S1 b, etc.

FIG. 9 shows an example of a double stimulus. FIG. 9 shows the stimuluswaveform 91 produced by inverter 3 includes a first portion in step S1 aand a second portion in step S1 b. In step S1 a, VDC is 6V, the durationis 206 μs and the switching frequency is 165 kHz. In step S1 b, VDC is6V, the duration is 60 s and the switching frequency is 200 kHz. Sincethe transmitter resonant frequency is approximately 100 kHz, thestimulus applied in step S1 a has a switching frequency closer to theresonant frequency, which provides relatively high energy input. In stepS1 b, the energy is reduced by increasing the switching frequency. Asshown, the rectifier filter capacitor Crec is fully charged before thestart of step S2, and the duration of step S1 is less than in theexample of FIG. 8. FIG. 9 also shows waveform 92 showing the currentthrough inductor L_(RES), waveform 93 showing the voltage of node Vres1,waveform 94 showing the current through rectifier filter capacitor Crec,waveform 95 showing the voltage at the input of the rectifier 14, andwaveform 96 showing the voltage across the rectifier filter capacitorCrec.

FIG. 10 shows an example of a double stimulus similar to FIG. 9, inwhich the energy is reduced in step S1 b by decreasing the voltage VDCrather than changing the switching frequency. In this example, VDC is 8Vin step S1 a and then is reduced to 6V in step S1 b.

FIG. 11 shows an example of a double stimulus similar to FIGS. 9 and 10in which the energy is reduced in step S1 b both by decreasing thevoltage VDC and changing the switching frequency in the way describedabove in FIGS. 9 and 10.

As discussed above, in step S2 the resonance of the transmitter isallowed to decay, and in step S3, a temporal characteristic of theresonance decay may be measured. For example, a decay time of theresonance decay may be measured, and/or the quality factor Q may bedetermined. The measurement of the temporal characteristic may beperformed using continuous time or discrete time measurements.

FIG. 12 shows an example of performing the measurement of step S3 usingcontinuous time measurements. A peak detector of controller 5 or aseparate peak detector may be used to detect the envelope of thedecaying waveform. As shown in FIG. 12, measurements V(t1) and V(t2) aremade at times t1 and t2, respectively. The quality factor Q may bedetermined using the following equations.

${{{For}\mspace{14mu} Q} > 10},{{{V(t)} = {{V(0)} \cdot {\exp \left\lbrack \frac{{- \omega} \cdot t}{2 \cdot Q} \right\rbrack}}};}$$\omega = \frac{2\pi}{T}$${Q = \frac{\pi \cdot \left( {t_{2} - t_{1}} \right)}{T \cdot {\ln \left\lbrack \frac{V\left( t_{2} \right)}{V\left( t_{1} \right)} \right\rbrack}}};$

FIG. 13 shows an example of determining Q using discrete timemeasurements. The peaks of the waveform as shown in FIG. 13 may bedetermined, then Q may be determined using the following equations.

${{{For}\mspace{14mu} Q} > 10},{{{V(n)} = {{V(0)} \cdot {\exp \left\lbrack \frac{{- 2}{\pi \cdot n}}{2 \cdot Q} \right\rbrack}}};}$$Q = \frac{\pi \cdot n}{\ln \left( \frac{V(n)}{V(0)} \right)}$

As discussed above, a multi-mode wireless power transmitter may becontrolled using controller 5, which may be implemented by any suitabletype of circuitry. For example, the controller 5 may be implementedusing hardware or a combination of hardware and software. Whenimplemented using software, suitable software code can be executed onany suitable processor (e.g., a microprocessor) or collection ofprocessors. The one or more controllers can be implemented in numerousways, such as with dedicated hardware, or with general purpose hardware(e.g., one or more processors) that is programmed using microcode orsoftware to perform the functions recited above.

In this respect, it should be appreciated that one implementation of theembodiments described herein comprises at least one computer-readablestorage medium (e.g., RAM, ROM, EEPROM, flash memory or other memorytechnology, or other tangible, non-transitory computer-readable storagemedium) encoded with a computer program (i.e., a plurality of executableinstructions) that, when executed on one or more processors, performsthe above-discussed functions of one or more embodiments. In addition,it should be appreciated that the reference to a computer program which,when executed, performs any of the above-discussed functions, is notlimited to an application program running on a host computer. Rather,the terms computer program and software are used herein in a genericsense to reference any type of computer code (e.g., applicationsoftware, firmware, microcode, or any other form of computerinstruction) that can be employed to program one or more processors toimplement aspects of the techniques discussed herein.

Various aspects of the apparatus and techniques described herein may beused alone, in combination, or in a variety of arrangements notspecifically discussed in the embodiments described in the foregoingdescription and is therefore not limited in its application to thedetails and arrangement of components set forth in the foregoingdescription or illustrated in the drawings. For example, aspectsdescribed in one embodiment may be combined in any manner with aspectsdescribed in other embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is:
 1. A foreign object detection method for a wirelesspower transmitter having a matching network and transmit coil, themethod comprising: measuring a quality factor of the transmit coil; andanalyzing the quality factor to determine whether a foreign object iscoupled to an electromagnetic field generated by the transmit coil,wherein the wireless power transmitter transfers sufficient energy to awireless power receiver to charge a rectifier filter capacitor of thewireless power receiver and reverse bias rectifier diodes of thewireless power receiver.
 2. The foreign object detection method of claim1, wherein the analyzing of the quality factor comprises comparing thequality factor to a threshold.
 3. The foreign object detection method ofclaim 2, wherein the threshold is at least in part derived from aquality factor value provided to the wireless power transmitter by thewireless power receiver.
 4. The foreign object detection method of claim1, further comprising determining whether to allow or inhibit wirelesspower transfer based on the analyzing.
 5. The foreign object detectionmethod of claim 1, wherein the measuring of the quality factor comprisesenergizing the matching network by transferring a first energy level ina first period of time and a second energy level in a second period oftime following the first period of time, wherein the first energy levelis higher than the second energy level.
 6. The foreign object detectionmethod of claim 5, wherein the measuring the quality factor furthercomprises allowing a resonance of the matching network to decay andmeasuring a temporal characteristic of the decay.
 7. A foreign objectdetection method for a wireless power transmitter having a matchingnetwork and transmit coil, the method comprising: energizing thematching network and transmit coil and exciting resonance between thematching network and transmit coil; measuring a quality factor of thetransmit coil; and comparing the quality factor to a threshold todetermine whether a foreign object is coupled to an electromagneticfield generated by the transmit coil.
 8. The foreign object detectionmethod of claim 7, wherein the threshold is at least in part derivedfrom a quality factor value provided to the wireless power transmitterby a wireless power receiver.
 9. The foreign object detection method ofclaim 7, further comprising: determining whether to allow or inhibitwireless power transfer based on the comparing of the quality factor tothe threshold.
 10. An apparatus for driving a wireless power transmitterand performing foreign object detection, the apparatus comprising: acontroller configured to measure a quality factor of a transmit coil ofthe wireless power transmitter and analyze the quality factor todetermine whether a foreign object is coupled to an electromagneticfield generated by the transmit coil, wherein wireless power transmittertransfers sufficient energy to a wireless power receiver to charge arectifier filter capacitor of the wireless power receiver and reversebias rectifier diodes of the wireless power receiver.
 11. The apparatusof claim 10, wherein the controller is configured to analyze of thequality factor by comparing the quality factor to a threshold.
 12. Theapparatus of claim 11, wherein the threshold is at least in part derivedfrom a quality factor value provided to the wireless power transmitterby the wireless power receiver.
 13. The apparatus of claim 10, whereinthe controller is configured to determine whether to allow or inhibitwireless power transfer based on analyzing the quality factor.
 14. Theapparatus of claim 10, wherein the controller is configured to measurethe quality factor through energizing a matching network by transferringa first energy level in a first period of time and a second energy levelin a second period of time following the first period of time, whereinthe first energy level is higher than the second energy level.
 15. Theapparatus of claim 14, wherein the controller is configured to allow aresonance of the matching network to decay and measure a temporalcharacteristic of the decay.
 16. An apparatus for driving a wirelesspower transmitter and performing foreign object detection, the apparatuscomprising: a drive circuit configured to energize a matching networkand transmit coil of the wireless power transmitter and excite resonancebetween the matching network and transmit coil; and a controllerconfigured to measure a quality factor of the transmit coil and analyzethe quality factor by comparing the quality factor to a threshold todetermine whether a foreign object is coupled to an electromagneticfield generated by the transmit coil.
 17. The apparatus of claim 16,wherein the threshold is at least in part derived from a quality factorvalue provided to the wireless power transmitter by a wireless powerreceiver.
 18. The apparatus of claim 16, wherein the controller isconfigured to determine whether to allow or inhibit wireless powertransfer based on the comparing of the quality factor to the threshold.